Flat-panel displays of various sizes are used widely for many computing and communication applications. In particular, flat-panel displays are used in both indoor and outdoor applications under a wide variety of ambient lighting conditions. For adequate visibility displays require an ambient contrast ratio of at least three for viewing text and at least 10 for viewing images or graphics, where ambient contrast ratio (ACR) is defined as:
                    ACR        =                  1          +                      PDL                          ADR              ×              AI                                                          (        1        )            where PDL is the peak display luminance in lumens per meter squared; ADR is percent average display reflectance; and AI is the ambient illumination in lux
Indoor applications experience relatively low levels of ambient illumination and require lower levels of display luminance to achieve the required ambient contrast ratio. Outdoor applications can experience relatively high levels of ambient illumination and may require higher levels of display luminance together with low display reflectance to achieve the required ambient contrast ratio. Moreover, most displays are preferably used in conditions of both high and low ambient illumination, from outdoor use during the day to nighttime use in a dark room.
Current illumination and display visibility standards cite 75,000 lux as a standard for outdoor ambient illumination on a bright and sunny day. Cloudy bright days have an ambient illumination of 16,000 lux and cloudy dull days have an ambient illumination of 6,000 lux. Indoor ambient illumination levels range from 0 to 1000 lux.
Conventional OLED displays have limited ambient contrast ratio and limited luminance. OLED displays have the problem that their ambient contrast ratio is greatly decreased when used in bright ambient illumination (due primarily to reflection of the ambient illumination from the face or substrate of the display). The luminance of an OLED display can be increased by increasing the current density flowing through the OLED light emissive materials, but doing so reduces the lifetime of the display, increases the power usage, and decreases the efficiency of the display. Moreover, at these higher currents, the power usage of the OLED displays is no longer competitive with back-lit LCD displays. It is known to use circular polarizers to increase the contrast of displays by reducing the reflection of ambient light from the display. Circular polarizers also decrease the light output from the display by about 60% while reducing the average display reflectance to 1.4%.
In an attempt to improve ambient contrast ratio in OLED displays, U.S. Pat. No. 6,274,980 by Burrows et al. proposes to increase the luminance of an OLED device by stacking multiple OLED layers, all of which emit the same light and utilize the same OLED materials. The electrodes of the stacked units are held in common, thus effectively creating an OLED display element whose luminance is increased by the number of light emissive elements in the stack. However, this approach does not increase the efficiency of the display and the increased power usage of the display may not be competitive with alternative display technologies, such as LCDs.
A variety of techniques have been proposed to improve the efficiency of OLED and other thin-film displays. For example, diffraction gratings have been proposed to control the attributes of light emission from thin polymer films by inducing Bragg scattering of light that is guided laterally through the emissive layers (“Modification of polymer light emission by lateral microstructure”, by Safonov, 2001, Synthetic Metals, 116, pp. 145–148 and “Bragg scattering from periodically microstructure light emitting diodes” by Lupton et al., Nov. 20, 2000 in Applied Physics Letters, vol. 77 Number 21 pp. 3340–3342). Brightness enhancement films having diffractive properties and surface and volume diffusers are described in WO0237568 A1 entitled “Brightness and Contrast Enhancement of Direct View Emissive Displays” by Chou et al., Mar. 2, 2001.
The use of micro-cavities and scattering techniques are also known, see for example “Sharply Directed emission in organic electroluminescent diodes with an optical-micro-cavity structure” by Tsutsui, et al., Oct. 10, 1994 in Applied Physics Letters 65, No. 15, pp. 1868–1870. However, these approaches still fail to meet the minimum ambient contrast ratio required for indoor and outdoor viewing.
It has been proposed to use a periodic, corrugated, grating structure to induce surface plasmon coupling for the light emitting layer in an organic luminescent device, thereby inhibiting lateral transmission and wave guiding of emitted light while increasing the efficiency and the light output of the structure. It is theoretically possible to couple up to 93% of the light emitted by the organic luminescent materials in an organic luminescent device. See Applied Physics Letter, Vol. 80, No. 20, entitled “Extraordinary transmission of organic photoluminescence through an otherwise opaque metal layer via surface plasmon cross coupling” by Gifford et al., May 20, 2002. Gifford et al. disclose creating the grating geometry for photoluminescent surface plasmon coupling by exposing a photoresist on glass with an interferometric pattern, followed by depositing subsequent layers that replicate the underlying surface profile. This approach is not compatible with the current manufacturing techniques used to make active matrix OLED displays, since for top emitting OLED display devices, a layer of thin film transistors are formed on the substrate prior to forming the OLEDs. For bottom emitting OLED displays, manufacturing starts with a glass substrate that is coated with a layer of conductive indium tin oxide (ITO) that is patterned to provide conductors for thin film transistors located on the substrate. The use of photoresist to create plasmon inducing gratings is problematical because the photoresist is an electrical insulator, thereby isolating the underlying ITO conductors from the OLED materials. Gifford et al. also suggest that the use of surface plasmon coupling can be an efficient means for outcoupling electroluminescence in an OLED device by using shadow masks on any desirable substrate. The use of shadow masks is not practical to create the gratings because of the small dimensions of the gratings. They also disclose that they have fabricated an OLED that uses surface plasmon coupling on a silicon substrate, but silicon substrates are not conventional or practical for OLED display devices.
There is a need therefore for an improved OLED display that avoids the problems noted above, while providing a minimum ambient contrast ratio for both indoor and outdoor viewing.